Modpack version
1.60

Can the issue be reproduced?
Yes

Issue
When I try to update te pack from the twitch client, it starts downloading but suddenly stops and revert. After that it shows this message: Unable to get Addon: CraftStudio API (268704) - File ID: 2661859

I've had this issue when updating from 1.59 to 1.60 and for me it seemed Twitch related. What I ended up doing is grabbing a backup from my personal files (journeymap and the like); doing a fresh install and pasting the personal files back in.

A lot of people on the public forums suggest to keep trying to update the modpack; every time it fails it will most likely fail on a different mod. Keep trying and eventually it pops through (source: https://www.reddit.com/r/feedthebeast/comments/67ahks/twitchcurse_app_mod_pack_downloading_error/)

reddit

Twitch/Curse App Mod pack downloading error

Is there anyone having problems downloading and installing modpacks through this new Twitch app thing, I'm trying to install FTB Beyond latest...

This error usually fixes itself, could you please try again?

I've tried a lot in the last week, but always the same error result. Today worked and the update goes fine... so nevermind :D
Thank you all

This error usually fixes itself, could you please try again?

the modpack didnt work and this happend:Failed to download file, the file contents differ from what was expected.
Name: jopt-simple-5.0.3.jar
URL: https://libraries.minecraft.net/net/sf/jopt-simple/jopt-simple/5.0.3/jopt-simple-5.0.3.jar
Filename on disk: jopt-simple-5.0.3.jar
Path: C:\Users\ALBIN\Twitch\Minecraft\Install\libraries\net\sf\jopt-simple\jopt-simple\5.0.3\jopt-simple-5.0.3.jar
Exists: File

Please don't attach your issue to an existing one, they're not the same. Try redownloading the pack, sounds like the Forge download messed up.